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Many types of fish undertake migrations on a regular basis, whether it is on a daily schedule to once per year. Physiological changes to their bodies take place to allow the switch in water type to occur. Distances traveled can range from a few yards to thousands of miles. Migrations are usually due to either feeding or breeding, but in some fish the reason for migration is still unknown.
Migratory fish are classified according to the following scheme:
DIADROMOUS - fish that travel between salt and fresh water.
There are three types of diadromous fish:
1) anadromous fish live in the sea, breed in fresh water.
2) catadromous fish live in fresh water and breed in the sea.
3) amphidromous fish move between fresh and salt water during
some part of life cycle, but not for breeding.
POTAMODROMOUS fish migrate within fresh water only.
OCEANODROMOUS fish migrate within salt water only.
Anadromous: Five salmon species including coho (silver) salmon; steelhead trout, smelt, shad (non-native to Pacific), most sturgeon and striped bass are common examples. Once the eggs of an anadromous fish hatch, the juvenile fish spend varying lengths of time in freshwater before migrating to saltwater, where they mature. The fish eventually return to freshwater to spawn. Some anadromous fish die after spawning (as with most salmon species), while others make the journey several times in their life. About half of all diadromous fish in the world are anadromous.
Catadromous: Sixteen variety of eels are in this category. Eels spend most of their adult lives in freshwater, and migrate to saltwater to spawn. Juvenile fish migrate back upstream where they stay until maturing into adults, at which time the cycle starts again. The only catadromous species in the United States is the American eel. The American eel migrates thousands of miles to spawn in the Sargasso Sea, located in the mid-Atlantic Ocean, south of Bermuda. Having spent so much energy to migrate and spawn, they die soon after.
Amphidromous: These fish move between estuaries and coastal rivers and streams, usually associated with the search for food and/or refuge rather than the need to reproduce. Amphidromous fish can spawn in either freshwater or in a marine environment. About one fifth of all diadromous fish are amphidromous.
Cutthroat trout, some sturgeon and catfish - these fish migrate within river systems or 2) is nonanadromous, residing solely in fresh water. The common belief is that these fish migrate short distances due to food availability or lack there of. Because the term Potamodromous has two common usages, the term has more commonly been replaced with the terms "freshwater migrants and nonmigrants" or the inclusive term "freshwater forms".
Herring, Tuna, sharks: Around 500 oceanodromous species migrate within international waters of the oceans. They comprise a wide variety of 104 different families. The most important ones are listed as "highly migratory species," this contains mackerels and tunas (Scombridae), all species of "pomfrets" (Bramidae), marlins (Makaira spp.), billfishes and sailfishes (Istiophoridae), swordfish (Xiphias gladius), sauries (Scomeberesocidae), dolphinfishes (Coryphaenidae), and 17 families of sharks and rays (Elasmobranchii). A considerable number of species are important target species for fisheries, and some have been the basis for entire economies, such as the herring (Clupea harengus) or the cod (Gadus morhua).
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